Python Flask and VirtualBox networking

I had been using the Python socket module to create a very basic client-server for testing purposes, but soon I wanted to have something slightly more standard, like an HTTP server. I decided to try the Python Flask framework.

First I set up a Flask server on a CentOS 7 Linux VM running on VirtualBox:

# yum install python-pip
# pip install Flask
# mkdir flask-server && cd flask-server

I created the hello.py file as described on the Flask homepage:

from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello World!"

Likewise, I started running Flask:

# FLASK_APP=hello.py flask run
 * Serving Flask app "hello"
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)

Then I set up port forwarding in VirtualBox on my desktop host so that I could communicate with the virtual machine, using the following settings:

Name: flask
Protocol: TCP
Host IP: 127.0.0.1
Host Port: 9500
Guest IP: 10.0.2.16
Guest Port: 5000

I tested it in a browser (Firefox) on my desktop at http://127.0.0.1:9500/

No connection. Firefox endlessly tries to load the file.

I tried from the local machine itself:

# curl http://localhost:5000/
Hello World!

I tried running tcpdump to see what the network traffic to that port looked like:

# tcpdump -n -i enp0s3 port 5000
...
14:54:11.938625 IP 10.0.2.2.63923 > 10.0.2.16.commplex-main: Flags [S], seq 3067208705, win 65535, options [mss 1460], length 0
...

Over and over I saw the same SYN packet from the client host, but the server never replied with a SYN-ACK.

I also noted that the local port was labeled commplex-main. This label is from /etc/services:

# grep commplex /etc/services
commplex-main   5000/tcp                #
commplex-main   5000/udp                #
commplex-link   5001/tcp                #
commplex-link   5001/udp                #

I don’t know what commplex-main is, but since I’m not running anything else on port 5000 other than Flask, it shouldn’t matter.

It turned out there were 2 separate problems:

1. Flask was listening only on localhost
2. firewalld was blocking the requests from external hosts

To fix the first, run Flask with the host flag:

# FLASK_APP=hello.py flask run --host=0.0.0.0
 * Serving Flask app "hello"
 * Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)

(This is mentioned in the Flask Quickstart guide, under Externally Visible Server.)

You can also specify an alternative port, e.g.:

# FLASK_APP=hello.py flask run --host=0.0.0.0 --port=56789
 * Serving Flask app "hello"
 * Running on http://0.0.0.0:56789/ (Press CTRL+C to quit)

To fix the latter temporarily, I disabled firewalld:

systemctl stop firewalld
systemctl disable firewalld

Obviously, if you are dealing with a machine connected directly to the Internet, this would be a terrible solution. You’d want to add rules allowing only the hosts and ports from which you expect to receive connections. But for testing communications between my desktop and a virtual host running on it, this seemed like a quick solution.

After those 2 changes, I was able to load the sample “hello” Flask app in a browser: